I accidentally deleted a calendar

gmsanta,

The first thing to try is Calendar>Edit>Undo. However, if you have executed any other actions in Calendar that selection will no longer be available, i.e. "grayed out."

If that selection is no longer available, use Time Machine to restore the Calendars Folder.

Quit Calendar, and find the correct Calendars Folder in the ~Library Folder. To find the correct Library Folder, paste this text ~/Library into the Finder Menu>Go>Go to Folder...window and select "Go." This will open the Users Library Folder where the Calendars Folder is located.

Keep that Library window open and replace Calendar from the proper Time Line.

Log out/in or restart and open Calendar.